Ceramics (oxygen ion conductors) having the property of causing selective permeation of oxygen ions at a high temperature (for example, 500° C. or higher) are known. Ceramic members formed from such oxygen ion conductors can be used with the object of separating oxygen from an oxygen-containing gas mixture. For example, an oxygen separation method using zirconium oxide as an oxygen ion conductor is known. In a representative modification of such a separation method, as shown in FIG. 11, external electrodes (not shown in the Figure) are pasted on both surfaces of a membranous ceramic member (oxygen permeable membrane) composed of zirconium oxide, and those electrodes are short circuited with an external circuit 116. This ceramic member 110 is disposed so that the partial pressure of oxygen at one surface side 110b of the membranous ceramic member 110 is lower than the partial pressure of oxygen on the other surface side 110a thereof. With such a configuration, on one surface 110a of the ceramic member 110, oxygen molecules accept electrons and become oxygen ions, and those oxygen ions diffuse (are conducted) in zirconium oxide and reach the other surface 110b where they discharge the electrons and become oxygen molecules. The discharged electrons are returned to the other surface 110a via the external circuit 116. As a result, oxygen is continuously separated from the gas, which is in contact with one surface 110a of the ceramic member 110. Technology of this type was disclosed in Japanese Patent No. 3,173,724 (Japanese Patent Application Laid-open No.H10-180031) and Japanese Patent Application Laid-open No.H9-299749.
On the other hand, some oxygen ion conductors demonstrate electron conductivity (the meaning of this term also includes hole conductivity), together with oxygen ion conductivity. Such oxygen ion conductors are also sometimes called electron—oxygen ion mixed conductors (hereafter referred to as “mixed conductors”). In the membranous ceramic members composed of such mixed conductors, as shown in FIG. 12, the ceramic member 120 itself has electron conductivity, and it is possible to cause a continuous permeation of oxygen ions from one surface 120a to the other surface 120b, without using external electrodes or an external circuit for short-circuiting the two surfaces. Technology of this type was openly disclosed in Japanese Patent Applications Laid-open Nos. 2001-106532, 2001-93325, 2000-154060, H 11-335164, H1 1-335165, H10-114520, and S56-92103, Japanese Patent No. 2,533,832 (Japanese Patent Application Laid-open No. H6-198149), Japanese Patent No. 2,813,596 (Japanese Patent Application Laid-open No. H6-219861), Japanese Patent No. 2,966,340 (Japanese Patent Application Laid-open No. H8-276112), Japanese Patent No. 2,966,341 (Japanese Patent Application Laid-open No. H9-235121), Japanese Patent No. 2,993639 (Japanese Patent Application Laid-open No. H11-253769), U.S. Pat. Nos. 5,306,411 and 5,356,728, Japanese Patent Application Laid-open Nos. 2001-269555, 2002-12472, and 2002-97083.
Examples of representative oxygen ion conductors include perovskite-type mixed conductors of the LaSrCoO3 type. Such conductors have a crystal structure in which part of La in a perovskite-type structure based on LaCoO3 is substituted with Sr. Furthermore, perovskite-type mixed conductors of the LaSrCoFeO3 type with a crystal structure, in which part of Co is replaced with a transition metal element such as Fe, have also been suggested. In conductors of such composition, the oxygen ion conductivity tends to increase, as the rate of substitution of La with Sr increases. However, in compositions with a high Sr substitution rate, when a membranous ceramic member composed of such a conductor is formed (fired), cracks easily appear in the ceramic member during use thereof (for example, when used as an oxygen permeable membrane). In particular, when such a conductor is exposed to a reducing atmosphere, the conductor is reduced. As a result, the crystal structure of the conductor changes and cracks can easily originate therein. The cracked ceramic member can no longer demonstrate its inherent performance (oxygen separation ability and the like). Thus, the ceramic member composed of a conductor with such a composition has poor endurance.
Examples of other representative oxygen ion conductors include mixed conductors having a perovskite-type structure of the LnGaO3 type (Ln is a lanthanoid). For example, a mixed conductor was suggested that had a crystal structure in which part of Ln in a perovskite-type structure based on LnGaO3 was substituted with an alkaline earth metal element such as Sr, and part of Ga was substituted with Fe. Such mixed conductors of the LnGaO3 type have high resistance to reduction (they are not easily reduced even when exposed to a reducing atmosphere, thereby maintaining their crystal structure). However, ceramic members formed from mixed conductors of the LnGaO3 type are relatively expensive due to the high cost of starting materials. Accordingly, there is a demand for ceramic members that have good endurance (resistance to reduction) and can be formed from an oxygen ion conductor that can be manufactured at a low cost.
On the other hand, the ceramic members formed from the aforesaid oxygen ion conductors can also be used in reactors for oxidation, for example for partial oxidation of hydrocarbons. For example, the ceramic member is formed into a membrane (this term includes also thin layers), one surface thereof is brought into contact with a gas containing oxygen, and the other surface is brought into contact with a gas containing a hydrocarbon (methane or the like). As a result, the hydrocarbon that is brought into contact with one surface of the ceramic member can be oxidized with oxygen ions that are supplied through the ceramic member from the other surface of the membranous ceramic member. In order to increase the efficiency of this oxidation reaction, a catalyst (Ni or the like) for enhancing the oxidation reaction can be applied to the first surface of the ceramic member. However, when a ceramic member formed from the conventional oxygen ion conductor (for example, of the LaSrCoO3 type, the LnGaO3 type, or the like) is used for partial oxidation of the hydrocarbons, some of the supplied hydrocarbons decompose on the first side of the ceramic member, and the catalyst easily degrades due to catalyst poisoning by the carbon that precipitates as a result of such decomposition.